1. Field of the Invention
The present invention relates to a semiconductor memory device. In particular, the present invention is directed to a bipolar-transistor type semiconductor memory device having a redundancy circuit portion for replacing a defective circuit portion such as a defective memory cell.
2. Description of the Related Art
In a random-access memory (RAM) device, a large number of memory cells are arranged along rows and columns. The density of defects generated in such a semiconductor memory device during the manufacture thereof is relatively independent of the integration density of the device. Rather, defects arise from the semiconductor manufacturing technology. In general, the higher the integration density of the device, the greater the ratio of normal memory cells to defective memory cells. This is one of the advantages of increasing the integration density of a semiconductor memory device. Even if a device includes only one defective memory cell, however, the device cannot be operated normally and, therefore, must be scrapped. As a result, despite the lower ratio of defective memory cells, greater integration density means reduced manufacturing yield.
In a metal-insulator-semiconductor (MIS) type RAM, in order to overcome the problem of defective memory cells, redundancy memory cells are employed. When a defective memory cell is detected, a redundancy memory cell row or column is selected instead of the memory cell row or column including the defective memory cell. In general, one or two redundancy memory cell rows or columns are usually provided.
In the prior art, however, there have been no bipolar-transistor type RAMs having a redundancy configuration, because there have been no redundancy configurations suitable for a bipolar-transistor type RAM, which operates at a much higher speed than a MIS type RAM. Also, there has not been any strong demand for a redundancy configuration for a bipolar-transistor type RAM, due to the comparatively small number of memory cells therein.
With improvements in integration technology for bipolar-transistor type RAMs and the requirement for more of memory cells, however, bipolartransistor type RAM devices having a large number of memory cells have come into demand. Accordingly, bipolar-transistor type RAMs having a redundancy configuration have come into demand.